U.S. patent number 4,883,723 [Application Number 07/211,019] was granted by the patent office on 1989-11-28 for hot dip aluminum coated chromium alloy steel.
This patent grant is currently assigned to Armco Inc.. Invention is credited to Richard A. Coleman, Frank C. Dunbar, Alan F. Gibson, Farrell M. Kilbane.
United States Patent |
4,883,723 |
Kilbane , et al. |
* November 28, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Hot dip aluminum coated chromium alloy steel
Abstract
Continuously hot dip aluminum coated ferritic chromium alloy
steel strip. After the steel has been given a pretreatment to
remove surface contaminants, the steel is protected in a hydrogen
atmosphere until it is passed into the molten aluminum coating
metal. The coating metal readily wets the steel surface to prevent
uncoated areas or pin holes in the coating layer.
Inventors: |
Kilbane; Farrell M.
(Centerville, OH), Coleman; Richard A. (West Chester,
OH), Dunbar; Frank C. (Middletown, OH), Gibson; Alan
F. (Middletown, OH) |
Assignee: |
Armco Inc. (Middletown,
OH)
|
[*] Notice: |
The portion of the term of this patent
subsequent to January 24, 2006 has been disclaimed. |
Family
ID: |
27360672 |
Appl.
No.: |
07/211,019 |
Filed: |
June 24, 1988 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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16920 |
Feb 20, 1987 |
4800135 |
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865238 |
May 20, 1986 |
4675214 |
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Current U.S.
Class: |
428/653; 427/432;
428/685; 427/320; 428/681; 428/939 |
Current CPC
Class: |
C23C
2/12 (20130101); Y10S 428/939 (20130101); Y10T
428/12951 (20150115); Y10T 428/12757 (20150115); Y10T
428/12979 (20150115) |
Current International
Class: |
C23C
2/12 (20060101); C23C 2/04 (20060101); B32B
015/18 (); B32B 015/20 () |
Field of
Search: |
;428/653,681,684,685,939
;427/320,432 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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245727 |
|
Dec 1985 |
|
JP |
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153255 |
|
Jun 1988 |
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JP |
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Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: Wyszomierski; George
Attorney, Agent or Firm: Bunyard; R. J. Fillnow; L. A.
Johnson; R. H.
Parent Case Text
This application is a continuation of application Ser. No. 016,920,
filed Feb. 20, 1987, now issued as U.S. Pat. No. 4,800,135 which is
a division of application Ser. No. 865,238, filed May 20, 1986, now
issued as U.S. Pat. No. 4,675,214.
Claims
We claim:
1. A ferrous base ferritic strip continuously hot dip coated with a
coating metal prepared by a process comprising:
cleaning the ferritic strip, the base metal of said cleaned strip
comprising at least about 6% by
weight chromium and less than about 3% by weight nickel, heating
said cleaned strip to a temperature near or slightly above the
melting point of said coating metal,
maintaining said cleaned strip in a protective atmosphere of at
least about 95% by volume hydrogen having a dew point of no more
than +40.degree. F. (+4.degree. C.) and containing no more than
about 200 ppm oxygen,
dipping said cleaned strip into a molten bath of said coating
metal, which is selected from the group consisting of aluminum and
aluminum alloy, to deposit a coating layer on at least one side of
said cleaned strip and said coating layer formed without a thick
brittle Fe-Al alloy inner layer,
said coating layer being substantially free of uncoated areas,
tightly adherent, and resistant to crazing and flaking during
bending of said continuous ferritic chromium alloy strip.
2. The strip as set forth in claim 1 wherein said base metal
includes at least about 10% by weight chromium.
3. The strips et forth in claim 2 wherein said base metal includes
substantially 0% by weight nickel.
4. The strip as set forth in claim 3 wherein said base metal
includes 10.0-14.5% by weight chromium, 0.1-1.0% by weight silicon,
and 0.2-0.5% by weight titanium.
Description
BACKGROUND OF THE INVENTION
This invention relates to a continuously hot dipped metallic coated
ferritic chromium alloy ferrous base strip and a process to enhance
the wetting of the strip surface with commercially pure molten
aluminum.
Hot dip aluminum coated steel exhibits a high corrosion resistance
to salt and finds various applications in automotive exhaust
systems and combustion equipment. In recent years, automotive
combustion gases have increased in temperature and become more
corrosive. For this reason, there has become a need to increase
high temperature oxidation resistance and salt corrosion resistance
by replacing aluminum coated low carbon or low alloy steels with
aluminum coated chromium alloy steels. For high temperature
oxidation and corrosion resistance, at least part of the aluminum
coating layer can be diffused into the iron base by the heat during
use to form an Fe-Al alloy layer. If uncoated areas are present in
the aluminum coating layer, accelerated corrosion leading to
perforation of the base metal may result if the Fe-Al alloy is not
continuously formed in the base metal.
It is well known to hot dip metallic coat steel strip without a
flux by subjecting the strip to a preliminary treatment which
provides a clean surface free of oil, dirt and iron oxide which is
readily wettable by the coating metal. Two types of preliminary
in-line anneal treatments for carbon steel are described in U.S.
Pat. No. 2,197,622 issued to T. Sendzimir and U.S. Pat. No.
3,320,085 issued to C. A. Turner, Jr.
The Sendzimir process for preparation of carbon steel strip for hot
dip zinc coating involves passing the strip through an oxidizing
furnace heated, without atmosphere control, to a temperature of
1600.degree. F. (870.degree. C.). The heated strip is withdrawn
from the furnace into air to form a controlled surface oxide. The
strip is then introduced into a reducing furnace containing a
hydrogen and nitrogen atmosphere wherein the residence time is
sufficient to bring the strip to a temperature of at least
1350.degree. F. (732.degree. C.) and to reduce the surface oxide.
The strip is then cooled to approximately the temperature of the
molten zinc coating bath and led through a snout containing a
protective pure hydrogen or hydrogen-nitrogen atmosphere to beneath
the surface of the coating bath.
The Turner process, normally referred to as the Selas process, for
preparation of carbon steel strip for hot dip metallic coating
involves passing the strip through a furnace heated to a
temperature of at least 2200.degree. F. (1204.degree. C.). The
furnace atmosphere has no fee oxygen and at least 3% excess
combustibles. The strip remains in the furnace for sufficient time
to reach a temperature of at least 800.degree. F. (427.degree. C.)
while maintaining a bright clean surface. The strip is then
introduced into a reducing furnace section having a
hydrogen-nitrogen atmosphere wherein the strip may be further
cooled to approximately the molten coating metal bath temperature
and led through a snout containing a protective hydrogen-nitrogen
atmosphere to beneath the surface of the coating bath.
U.S. Pat. No. 3,925,579 issued to C. Flinchum et al. describes an
inline pretreatment for hot dip aluminum coating low alloy steel
strip to enhance wettability by the coating metal. The steel
contains one or more of up to 5% chromium, up to 3% aluminum, up to
2% silicon and up to 1% titanium. The strip is heated to a
temperature above 1100.degree. F. (593.degree. C.) in an atmosphere
oxidizing to iron to form a surface oxide layer, further treated
under conditions which reduce the iron oxide whereby the surface
layer is reduced to a pure iron matrix containing a uniform
dispersion of oxides of the alloying elements.
It is well know that hot dip aluminum coatings do not wet cleaned
steel surfaces as easily as zinc coatings. U.S. Pat. No. 4,155,235
to Pierson et al. discloses the importance of keeping hydrogen gas
away from the entry section of an aluminum coating bath. This
patent teaches a cleaned steel must be protected in a nitrogen
atmosphere just prior to hot dip aluminum coating to prevent
uncoated spots.
The problems associated with non-wetting of aluminum coatings onto
ferritic stainless steel are also well known. Hot dip aluminum
coatings are poorly adherent to ferritic stainless steel base
metals and normally have uncoated or bare spots in the aluminum
coating layer. By poor adherence is meant flaking or crazing of the
coating during bending of the strip. To overcome the adherence
problem, some have proposed heat treating the aluminum coated
stainless steel to anchor the coating layer to the base metal.
Others lightly reroll the coated stainless steel to bond the
aluminum coating. Finally, those concerned about uncoated spots
have generally avoided continuous hot dip coating. Rather, batch
type hot dip coating or spray coating processes have been used. For
example, after a stainless steel article has been fabricated, it is
dipped for an extended period of time within an aluminum coating
bath to form a very thick coating layer.
No one has proposed a solution for enhancing the wetting of
ferritic chromium alloy steels using hot dip aluminum coatings.
Without good surface wetting, the aluminum coating layer will not
be uniform, free of uncoated areas and strongly adherent to the
steel base metal. We have discovered a coating method for
overcoming the wetting problems associated with hot dip aluminum
coating of ferritic chromium alloy steel. The wetting is
dramatically improved if a cleaned ferritic chromium alloy steel is
maintained in a protective hydrogen atmosphere substantially void
of nitrogen prior to the entry of the steel into an aluminum
coating bath.
BRIEF SUMMARY OF THE INVENTION
This invention relates to a continuous hot dip aluminum coated
ferrous base ferritic steel containing at least about 6% by weight
chromium. The surface of the steel is pretreated to remove oil,
dirt, oxides and the like. The steel is then heated to at least
1250.degree. F. (677.degree. C.) and then protected in an
atmosphere containing at least about 95% by volume hydrogen with
the steel being maintained at a temperature near or slightly above
the melting point of a coating metal consisting essentially of
aluminum. The hydrogen atmosphere enhances the wetting of the
ferritic chromium steel to substantially eliminate uncoated or pin
hole defects in the aluminum coating layer.
It is a principal object of this invention to form hot dip aluminum
coated ferritic chromium alloy steels having enhanced wetting by
the coating metal.
An advantage of our invention is elimination of uncoated areas and
improved adherence to ferritic chromium alloy base metals when hot
dip coating with aluminum.
Another advantage of our invention is improved high temperature
oxidation and salt corrosion resistance thereby increasing base
metal perforation resistance for aluminum coated ferritic chromium
alloy steels used in automotive exhaust systems.
The above and other objects, features and advantages of this
invention will become apparent upon consideration of the detailed
description and appended drawings.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of a ferrous base strip being processed
through a conventional hot dip aluminum coating line incorporating
the present invention;
FIG. 2 is a partial schematic view of the coating line of FIG. 1
showing an entry snout and coating pot.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, reference numeral 10 denotes a coil of
steel with strip 11 passing therefrom and around rollers 12, 13 and
14 before entering the top of first furnace section 15. This first
section of furnace 15 may be a direct fired type having
approximately 5 percent excess of combustibles introduced therein.
The furnace atmosphere temperature may be on the order of
2300.degree. F. (1260.degree. C.). Strip surface contaminants such
as oil and the like are almost instantaneously burned and
removed.
The second section of the furnace denoted by numeral 16 may be of a
radiant tube type. The temperature of strip 11 may be further
heated to about 1250.degree. F. (677.degree. C.) to 1750.degree. F.
(954.degree. C.) and reaching a maximum temperature at about point
18. A reducing atmosphere will be supplied to section 16 as well as
succeeding sections of the furnace described below. The atmosphere
must be as reducing, and preferrably more so, than that used for
carbon steels to minimize oxidation of chromium in the base
metal.
The third section of the furnace generally denoted by numeral 10 is
a cooling zone.
The final section of the furnace generally denoted by numeral 22 is
a final cooling zone. Strip 11 passes from furnace portion 22, over
turndown roller 24, through snout 26 and into coating pot 28
containing molten aluminum. The strip remains in the coating pot a
very short time (i.e., 2-5 seconds). Strip 11 containing a layer of
coating metal is vertically withdrawn from coating pot 28. The
coating layer is solidified and the coated strip is passed around
turning roller 32 and coiled for storage or further processing in
coil 34.
Referring now to FIG. 2, snout 26 is protected from the atmosphere
by having its lower or exit end 26a submerged below surface 44 of
aluminum coating metal 42. Suitably mounted for rotation are pot
rollers 36 and 38 and stabilizer roller 40. The weight of coating
metal 42 remaining on strip 11 as it is withdrawn from the coating
pot is controlled by a coating means such as jet finishing knives
30. Strip 11 is cooled to a temperature near or slightly above the
melting point of the aluminum coating metal in furnace portions 20,
22 and snout 26 before entering the coating pot. This temperature
may be as low as about 1220.degree. F. (660.degree. C.) to as high
as about 1350.degree. F. (732.degree. C.).
The process thus far described is well known in the art and is for
two side coating using air finishing. As will be understood by
those skilled in the art, modifications to the pretreatment process
for cleaning the strip surface may be used such as using wet
cleaning instead of the direct fired furnace. Furthermore, it will
be understood by those skilled in the art one-side hot dip coating
or finishing using a sealed enclosure containing a non-oxidizing
atmosphere may be used with this invention.
Referring to FIG. 2, our invention will be described in detail. To
enhance the wetting of a hot dip aluminum coating metal to steel
strip containing a ferritic alloy of at least about 6% by weight
chromium, the steel strip is given a suitable pretreatment to
remove dirt, oil film, oxides and the like. The strip is further
heated in an atmosphere reducing to iron such as containing 20% by
volume hydrogen and 80% by volume nitrogen and thereafter passing
the cleaned strip through a protective atmosphere of substantially
all hydrogen just before entering the coating bath. When an in-line
annealing such as described above is used to clean the strip, the
protective atmosphere is maintained in an enclosure such as
enclosed snout 26. Hydrogen gas can be introduced as necessary such
as through inlets 27. The protective atmosphere must contain at
least about 95%, more preferably at least 97%, and most preferably
as close to 100% as possible, by volume hydrogen.
It is also very important to control oxygen and dew point of the
protective atmosphere as well as maintaining a high molten metal
temperature in the coating pot. A thin oxide layer on the surface
of a steel strip may be reduced by the reactive aluminum coating
metal. Chromium is much more readily oxidized than iron so that
chromium alloy steels are more likely to be non-wetted because of
excessively thick oxide films than carbon steels. Accordingly, the
protective hydrogen atmosphere must have a few point no higher than
about +40.degree. F. (4.degree. C.) and containing no more than
about 200 ppm oxygen. Preferably, the dew point should be less than
+10.degree. F. (-12.degree. C.) and oxygen less than 40 ppm.
Substantially pure aluminum coating metals are normally maintained
at about 1250.degree. F. (677.degree. F.) to 1270.degree. F.
(688.degree. C.) for coating carbon steel. Because of the increased
tendency for chromium alloy steels to oxidize, we must maintain our
coating metal at least this high and preferably in the range of
1280.degree. F. (693.degree. C.) to 1320.degree. F. (716.degree.
C.). This increased temperature increases the reactivity of the
coating metal making it more reducing to chromium oxide. The
temperature should not exceed about 1320.degree. F. (716.degree.
C.) because an excessively thick brittle Fe-Al alloy layer may
form.
The present invention has particular usefulness for hot dip
aluminum coated ferritic stainless steels used in automotive
exhaust applications, including thin foils used as supports for
catalytic converters. This later steel is described in co-pending
application filed June 4, 1985 under U.S. Ser. No. 741,282 now
issued as U.S. Pat. No. 4,686,155 and assigned to a common
assignee. A ferritic stainless steel containing at least about 10%
by chromium having a hot dip coating of substantially pure aluminum
will have excellent corrosion resistance. Unlike aluminum coated
carbon steel, we have discovered that a ferritic stainless steel
hot dip coated with pure aluminum may be severely fabricated
without flaking or crazing the coating layer. It has been
determined a Type 409 stainless steel containing about 10.0% to
about 14.5% by weight chromium, about 0.1% to about 1.0% by weight
silicon, about 0.2% to about 0.5% titanium and the remainder iron
may be hot dip coated with pure aluminum. Furthermore, the coated
strip may be cold reduced from strip of at least 0.25 mm thickness
to less than 0.1 mm without peeling the coating metal. Because the
aluminum coating layer has excellent adherence to the base metal
and does not contain pin hole or uncoated areas, a diffusion heat
treated foil has excellent oxidation resistance at high
temperatures. For example, the foil may be used as catalyst
supports in automotive exhausts having operating temperatures of
about 1500.degree. F. (800.degree. C.)-1650.degree. F. (900.degree.
C.) with "brief excursions" as high as 2200.degree. F.
(1204.degree. C.).
In addition to carbon and low alloy steels, chromium alloy steels
containing substantial amounts of nickel are readily hot dip
aluminum coated using conventional practice. By substantial amount
of nickel is meant in excess of about 3% by weight such as
austenitic stainless steels. Chromium alloy steels containing 3% or
more nickel apparently are easily coated with aluminum because the
nickel appears to form a very tight bond with the aluminum.
Accordingly, these high nickel chromium alloy steels may be readily
hot dip coated with aluminum without using our invention. Most hot
dip aluminum coatings contain about 10% by weight silicon. This
coating metal is generally defined in the industry as Type 1. We
have discovered this type aluminum coating metal does not wet well
with ferritic chromium alloy steel, even when using the hydrogen
protective atmosphere. While not being bound by theory, it is
believed silicon exceeding 0.5% by weight decreases the reactivity
of the aluminum coating metal needed to react with a ferritic
chromium alloy steel substrate. Accordingly, silicon contents in
the coating metal should not exceed about 0.5% by weight.
Commercially pure hot dip aluminum coatings, otherwise known as
Type 2 in the industry, are preferred for our invention. By "pure"
aluminum is meant those aluminum coating metals where addition of
substantial amounts of alloying elements, such as silicon, are
precluded. It will be understood that coating metal may contain
residual amounts of impurities, particularly iron. The coating bath
typically contains about 2% by weight iron caused primarily by
dissolution of iron from the steel strip passing through the
bath.
EXAMPLE 1
To illustrate the inability to prevent uncoated areas when using a
conventional protective atmosphere, 3 inch wide (76 mm) strip of
409 stainless was given an in-line anneal pretreatment on a
laboratory pilot line. The direct fired portion of the furnace was
heated to about 2150.degree. F. (117520 C.) and the strip peak
metal temperature observed was about 1650.degree. F. (899.degree.
C.). The strip was cooled to about 1285.degree. F. (696.degree. C.)
in the snout just prior to entry into the aluminum coating
bath.
The steel strip was protected in the snout portion of the furnace
using a protective atmosphere containing about 25% by volume
hydrogen and the balance nitrogen with a dew point less than
-15.degree. F. (-26.degree. C.) and less than 40 ppm oxygen. The
aluminum coating metal in the coating pot was maintained at about
1285.degree. F. (696.degree. C.). The as-coated strip contained an
estimated uncoated area of about 25% and occasionally was as high
as 75%.
EXAMPLE 2
To demonstrate the enhanced wetting when using a protective
atmosphere according to the invention, a 3 inch (76 mm) wide strip
of 409 stainless steel was coated on the same pilot line and was
given an in-line anneal pretreatment having temperatures similar to
those set forth in Example 1. However, the atmosphere was adjusted
to include about 100% by volume hydrogen, -15.degree. F.
(-26.degree. C.) dew point and less than 40 ppm oxygen. The
as-coated strip appearance was excellent and no visible uncoated
areas or pin holes were apparent.
EXAMPLE 3
A 3 inch (76 mm) strip of 409 stainless steel was coated on the
pilot line. The strip was heated to a peak metal temperature of
1600.degree. F. (871.degree. C.) and was cooled to 1280.degree. F.
(693.degree. C.) in the snout just prior to entry into the aluminum
coating bath. The atmosphere contained a dew point of -15.degree.
F. (-26.degree. C.) and 20 ppm oxygen. A gas chromatograph was
installed in the snout so that strip as-coated coating quality
could be observed as the amount of hydrogen in the protective
atmosphere was varied. When the atmosphere was about 92% by volume
hydrogen and the balance nitrogen, the coating quality was
unacceptable. Increasing the hydrogen to about 94% by volume
produced what was considered to be marginally acceptable coating
quality. When the hydrogen was increased to 97% by volume, the
coating quality observed was considered to be excellent and the
coating layer had substantially no uncoated areas.
A trial was also run on a product size hot dip aluminum coating
line. The following temperature--atmosphere conditions were used
and coating quality observations made:
__________________________________________________________________________
DFF* Temp. Peak Metal Ex. .degree.F. (.degree.C.) Temp. .degree.F.
(.degree.C.) Pot Temp. .degree.F.(.degree.C.) Dew Point .degree.F.
(.degree.C.) % Hydrogen Observation
__________________________________________________________________________
4. 1040 (560) 1400 (760) 1270 (687) +7 (-14) 0 50% uncoated 5. 1040
(560) 1400 (760) 1270 (687) +7 (-14) 100 no uncoated 6. 1300 (704)
1600 (871) 1280 (693) +25 (-4) 100 15% uncoated 7. 1300 (704) 1600
(871) 1300 (704) +30 (-1) 100 no uncoated
__________________________________________________________________________
*Strip temperature in the direct fired furnace section
Various modifications can be made to our invention without
departing from the spirit and scope of it. For example, various
modifications may be made to the protective atmosphere so long as
it includes at least about 95% by volume hydrogen. Furthermore,
modifications may be made to the strip pretreatment as well as
using one-side coating or non-oxiding jet finishing. Therefore, the
limits of our invention should be determined from the appended
claims.
* * * * *